System, method and apparatus

ABSTRACT

The present invention relates to an apparatus, system or method for reducing pressure in a gas flow for a gas let-down system. The present invention further relates to an apparatus, system or method for drying gas. A system ( 10 ) for reducing pressure in a gas flow for a gas let-down system comprises an expander ( 102 ) driven by gas at a first pressure expanding to a second pressure, and a compressor ( 104 ) for compressing the gas from the second pressure to a third pressure, whereby the third pressure is lower than the first pressure and the third pressure is higher than the second pressure. By first expanding the gas and then compressing the gas the intermediate temperature of the gas at the second pressure is lower than if the gas is expanded directly to the third pressure. Further, a drying system for drying a gaseous fluid supplying heat to a heat exchanger comprises a liquid separator, a heat exchanger downstream of the liquid separator and a cooler for extracting heat from the gaseous fluid upstream of the liquid separator using the cold gas downstream of the heat exchanger. By extracting heat from the gaseous fluid the temperature of the gaseous fluid at the inlet to the separator can be reduced causing liquid in the gaseous fluid to condense and be separated in the separator.

The present invention relates to an apparatus, system or method for reducing pressure in a gas flow for a gas let-down system. The present invention further relates to an apparatus, system or method for drying gas.

The present invention relates to a system for reducing pressure in a gas flow for a gas let-down system. According to one aspect of the invention, there is provided a system for reducing pressure in a gas flow for a gas let-down system comprising an expander driven by gas at a first pressure expanding to a second pressure, and a compressor for compressing the gas from the second pressure to a third pressure, whereby the third pressure is lower than the first pressure and the third pressure is higher than the second pressure. By first expanding the gas and then compressing the gas the intermediate temperature of the gas at the second pressure is lower than if the gas is expanded directly to the third pressure. The gas is preferably natural gas. The expander is preferably a turbine. The combination of an expander and a compressor may also be referred to as a compander.

Preferably the expander drives the compressor, preferably directly. This can provide efficiency. By directly is preferably meant without conversion to another energy form such as electricity, pressure or heat. Preferably the expander drives the compressor by way of a common shaft. Optionally a mechanical coupling may connect the expander and the compressor.

Preferably the system further comprises a heat exchanger for heating the gas at the second pressure. This can allow efficient heating of the gas. Preferably the heat exchanger is arranged to provide heat exchange to ambient air. The heat exchanger may be arranged to provide heat exchange to a secondary circuit for absorbtion of heat from ground or water or other ambient source or a waste heat source. The heat exchanger may be arranged to provide cooling to a refrigeration load. The heat exchanger may be arranged to provide heat exchange to ground, or water, or an ambient heat source, or a waste heat source. The system may further comprise a plurality of heat exchangers, with each of the heat exchangers arranged to provide heat from a different heat source. A secondary circuit may be arranged to transfer heat from a heat source to the or a heat exchanger. The cold gas at the outlet of the expander may transfer energy via a secondary circuit or a heat pipe to provide cooling to a refrigeration load.

Preferably the expander further drives an electric generator. The electricity may be used to drive further electrical parts such as a fan and/or an electric defrosting heater. The electricity may be used for exportation to an electrical grid.

Preferably the system is arranged to conduct a portion of gas to the inlet of the expander, and to conduct a further portion of gas to the outlet of the compressor, the further portion of gas bypassing the expander and the compressor. The further portion of gas may drive a further expander that optionally further drives an electric generator. The further portion of gas may be expanded at a valve. The system may provide sufficient excess heat in the compander outlet gas stream to be mixed with another stream (such as the further portion of gas) from a valve or an expander with an electrical output to produce a mixed gas stream at a suitable temperature for the downstream requirements.

Preferably the system is arranged to conduct a portion of gas to the inlet of the expander, and to conduct a further portion to the compressor, the further portion bypassing the expander. The further portion preferably drives the compressor, preferably by means of a tip turbine. The compressor preferably comprises a tip turbine. This can provide supplementary drive to the compressor.

Preferably the system is arranged to conduct a portion of gas to the inlet of the expander, and to conduct a further portion of gas to an inlet of a further expander. Preferably the further expander and the expander both drive the compressor. Preferably the system is arranged to conduct the further portion of gas from the outlet of the further expander to the outlet of the compressor.

Preferably the system further comprises a recuperator for transferring heat from one portion of the gas to another. Preferably the recuperator is arranged to transfer heat from gas upstream of the expander to gas downstream of the heat exchanger. This can serve to reduce the gas temperature at the heat exchanger to a sufficiently low value to enable effective heat exchange with ambient air (or a secondary circuit for absorbtion of heat from ground or water or other ambient source or a waste heat source). This may be appropriate if the temperature drop and pressure drop across the expander are relatively small.

The recuperator may be arranged to transfer heat from gas downstream of the compressor to gas upstream of the expander. This can pre-heat the gas entering the expander. The recuperator can prevent the gas downstream of the expander from excessive cooling which can cause solid formation and frosting at the heat exchanger, leading to deterioration of the performance of and potentially damage to the system. Also, gas exiting the compressor can be provided at a high enough temperature for effective defrosting of the heat exchanger in case of solids formation occurring.

Preferably the system is arranged to conduct a portion of gas from the outlet of the compressor to the inlet of the heat exchanger. This can provide additional heating and prevent frosting of the heat exchanger. Different sections may be defrosted sequentially at different times. Preferably the system is arranged to conduct a portion of gas from the outlet of the compressor to the inlet of the compressor. This can enable adjustment of the duty position of the system by increasing compressor power when open.

Preferably the system further comprises a sealable vessel containing system rotative components. Preferably the system rotative components include at least an output drive shaft of the expander and an input drive shaft of the compressor.

Preferably the system rotative components include the common shaft by way of which the expander drives the compressor. Preferably the sealable vessel contains the generator, expander and compressor. The sealable vessel can enable the entire rotative system to operate in an environment at one of the system gas flow pressures, thus avoiding the requirement for rotating seals in communication with the external environment, therefore removing the risk of natural gas leakage and potential explosion.

Preferably the system further comprises a gas and/or magnetic bearing supporting the output drive shaft of the expander and/or the input drive shaft of the compressor. Gas and/or magnetic bearings can enable particularly low contamination of the gas in the expander and the compressor, in particular with respect to oil lubricant, as they require no oil lubricant.

Preferably the system further comprises a controller for activating the system when an ambient air temperature is below a pre-defined threshold. This can enable the system to operate selectively when the ambient environment makes it necessary, and otherwise assume another mode of operation such as in a conventional let-down gas expander. The system can assist when the ambient temperature (and in particular the temperature of the incoming gas which is typically at ground temperature) is too low to permit reducing pressure in a gas flow directly from the first to the third pressure, as in a conventional let-down gas expander. When the ambient temperature is high enough to permit reducing pressure in a gas flow directly from the first to the third pressure the system can be deactivated. The controller may also activate the system for a pre-determined period of the year, preferably during a period of the year that is expected to have an ambient air temperature below a threshold (for example in winter months). Alternatively, if the system is operated when an ambient air temperature is above a pre-defined threshold, then the generator is able to generate more electricity. In this example the system may further comprise a controller for exporting surplus electricity when an ambient air temperature is above a pre-defined threshold.

Preferably the system further comprises a drying system as described below.

Features of the system may include:

-   -   Turbine directly drives compressor     -   For use in natural gas let-down of compressed natural gas     -   Process gas is used as the operating fluid for the heat pump

According to a further aspect of the invention, there is provided a drying system for drying a gaseous fluid (preferably air) supplying heat to a heat exchanger, the drying system comprising a liquid separator (for separating a liquid from the gaseous fluid); a heat exchanger (for transferring heat away from the gaseous fluid) downstream of the liquid separator; and a cooler for extracting heat from the (warm) gaseous fluid upstream of the liquid separator using the (cold) gaseous fluid downstream of the heat exchanger.

By extracting heat from the gaseous fluid the temperature of the gaseous fluid at the inlet to the separator can be reduced causing liquid in the gaseous fluid to condense and be separated in the separator (and thus removed from the gaseous fluid). This can prevent excessive liquid from entering the heat exchanger, which can cause frosting and lead to poor performance of and damage to the heat exchanger.

Preferably the cooler is a recuperator for transferring heat from gaseous fluid upstream of the liquid separator to gaseous fluid downstream of the heat exchanger. A recuperator can enable efficient transfer of heat.

Preferably the cooler is a mixer for mixing a portion of gaseous fluid from downstream of the heat exchanger with gaseous fluid upstream of the liquid separator. A mixer can provide a simple implementation of the cooler.

The system may comprise a controller for controlling the portion of gaseous fluid conveyed from downstream of the heat exchanger back upstream of the liquid separator. The controller may comprise a fan for establishing a flow rate of the conveyed portion. The controller may respond to a signal from a detector. The detector may detect frosting or a liquid loading of the gaseous fluid.

Preferably the gaseous fluid comprises a component that is in equilibrium with a liquid at the inlet to the heat exchanger, and in equilibrium with a solid at the outlet of the heat exchanger. Condensing and separating liquid from the gaseous fluid can be particularly beneficial in cases where the gaseous fluid condition changes due to the heat exchange such that a solid can form. This can be in particular a change (decrease) of temperature within the heat exchanger. The liquid may be water and the solid may be ice. The liquid may be a hydrocarbon.

The separator may be a gravitational separator, a vortex separator or a plate separator.

According to a further aspect of the invention, there is provided apparatus for gas let-down comprising an expander driving a compressor. Preferably the expander comprises a turbine.

According to a further aspect of the invention, there is provided a system for drying a wet gas (preferably air) for supplying heat to a heat exchanger, the system comprising a liquid separator upstream of a heat exchanger and a conduit for conveying a portion of gas from downstream of a heat exchanger back upstream of the liquid separator, and re-introducing that portion of gas.

According to a further aspect of the invention, there is provided a gas let-down station comprising a system for reducing pressure in a gas flow as described above.

According to a further aspect of the invention, there is provided a gas distribution network comprising a system for reducing pressure in a gas flow as described above.

The invention extends to methods and/or apparatus substantially as herein described with reference to the accompanying drawings.

Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

These and other aspects of the present invention will become apparent from the following exemplary embodiments that are described with reference to the following figures in which:

FIG. 1 shows an embodiment of a system for reducing pressure in a gas flow;

FIG. 2a shows a further embodiment of a system for reducing pressure in a gas flow;

FIG. 2b shows a further embodiment of a system for reducing pressure in a gas flow;

FIG. 2c shows a further embodiment of a system for reducing pressure in a gas flow;

FIG. 3 shows an embodiment of a system for reducing pressure in a gas flow with a low let-down pressure drop;

FIG. 4 shows a further embodiment of a system for reducing pressure in a gas flow with a low let-down pressure drop;

FIG. 5 shows an embodiment of a system for reducing pressure in a gas flow with a high let-down pressure drop;

FIG. 6 shows an embodiment of a system for reducing pressure in a gas flow with an upstream separator;

FIG. 7 shows a drier for a heat exchanger; and

FIG. 8 shows a further drier for a heat exchanger.

At a conventional gas let-down station gas (natural gas) pressure is reduced by expansion across a valve. Following expansion the outlet temperature can be too low to allow the gas to be reintroduced to the downstream pipe network, so the gas needs to be heated to offset the cooling effect from the expansion of gas. Conventionally gas is burned to provide this heat, with both an economic and an environmental cost. Water bath heating, as is conventionally used for heating expanded gas at let-down stations, uses significant quantities of gas and is expensive to run and maintain. Heating by burning gas is also environmentally damaging due to the CO₂ release from combustion.

It has been proposed to use the expansion energy to generate electricity and to use this electricity to run a vapour compression heat pump to heat the gas. This is an expensive and complex way to heat the gas.

A gas let-down system with an expander and combined heat pump that uses the expansion energy from a turbine to directly drive a compressor which acts as a heat pump is proposed. The intermediate pressure of the gas, downstream of the turbine and upstream of the compressor, is lower than the outlet pipeline pressure. At the same time the intermediate temperature of the gas is lower than it would be if the gas were brought directly to the outlet pipeline pressure. The combination of the expander and the compressor allows the turbine to work over a greater pressure ratio and increases the temperature drop of the gas. Ordinarily in gas let-down the outlet gas temperature is too high to allow effective heat exchange with the environment. Due to the small temperature difference, in the order of for example only a few degrees Celsius, the size of the heat exchanger (in the order of magnitude of acres) would be prohibitive. Heat exchange may not even be possible if the air temperature is too low. By the combination of the expander and the compressor a greater temperature difference is available, for example 20° C. or more. With the increased gas temperature drop, efficient heat exchange with the environment at the cold intermediate condition can heat the gas. The size of the heat exchanger can be in the order of tens of square metres rather than the prohibitively sized heat exchangers that would be required in conventional systems. Instead of requiring burning of gas to heat the gas, heat exchange with the environment can raise the gas temperature sufficiently.

This can achieve a number of things:

-   -   1. Increase the temperature difference with ambient air (or         other suitable ambient or waster heat sources) to allow smaller         heat exchangers.     -   2. Allow some pre-heat to be applied to the gas.     -   3. Move the position on the psychrometric chart to reduce the         propensity of the turbine to ice up (in particular control the         pressure and temperature in order to decrease the relative         humidity of gas passing through the turbine).     -   4. Provide heat to defrost external heat exchangers.

The system harvests heat from the atmosphere (or other suitable ambient or waster heat sources) and uses this to pre-heat the gas, avoiding the need for gas combustion and water bath heaters. The combination of expander and compressor provides outlet gas at the required pressure and temperature for the downstream network. The system has aspects of both expander and heat pump, taking atmospheric heat and boosting it to a temperature suitable for the efficient heating of gas within the process to acceptable outlet temperatures.

FIG. 1 shows a basic system 10 for reducing pressure in a gas flow 120. The gas flow 120 is directed first to an expander 102, typically a turbine (but other expanders such as a screw expander are possible), and subsequently to a compressor 104. The intermediate pressure of the intermediate gas flow 128, downstream of the turbine 102 and upstream of the compressor 104, is lower than the outlet pipeline pressure of the outlet gas flow 126. The intermediate temperature of the intermediate gas flow 128 is lower than if the gas pressure were at the outlet pipeline pressure. At the heat exchanger 108 atmospheric air provides heat to warm up the intermediate gas flow 128. The warmed intermediate gas flow 128 is directed to a compressor 104 that increases the gas pressure; downstream of the compressor 104 the system outlet gas flow 126 is at the outlet pipeline pressure. The outlet pipeline pressure of the outlet gas flow 126 is below the inlet pipeline pressure of the inlet gas flow 120; and the intermediate pressure of the intermediate gas flow 128 is below the outlet pipeline pressure of the outlet gas flow 126.

The compressor and turbine are relatively balanced with 1-2 MW shaft power transferred.

In FIG. 1 the heat exchanger 108 is arranged to warm up the intermediate gas flow 128 where atmospheric air provides heat. In an alternative, such an air source heat pump is replaced with a ground source heat pump, such that a ground source air provides heat to warm up the intermediate gas flow 128. Other heat sources for warming up the intermediate gas flow 128 can be used as available and convenient.

FIG. 2a shows a variant of the system shown in FIG. 1, where the cold gas at the outlet of the expander (the intermediate gas flow 128) is arranged to provide cooling to a refrigeration load by transferring a portion of the energy away from the cold gas. The transfer of energy for refrigeration purposes can be arranged via a heat exchanger 121 to where heat is absorbed from a secondary circuit 123 as shown in FIG. 2a . Alternative means for the transfer of energy, such as a heat pipe for example may be used.

The system can be configured to additionally generate electricity. The electrical generation is 100 kW in an example, which provides enough electricity to operate fans for the system. An electric defrost system (in particular for low temperature portions of the gas flow) can also or alternatively be operated by the electricity.

Systems configured to also be operable during summer months can convert a significant proportion of the turbine power to electrical power due to the reduced necessity to use the heat pump features of the system, with up to 1 MW able to be exported.

FIG. 2b shows a system 20 for reducing pressure in a gas flow 120 having an electric generator 110. The turbine 102 drives both the compressor 104 and also the electric generator 110. The electricity generated by the electric generator 110 is used to power a fan 112 that assists the heat exchanger 108.

Further, a pressure envelope 130 encloses the turbine 102, the compressor 104 and the generator 110. The pressure envelope 130 includes suitable connections for inlet and outlet of gas streams and also for electrical connection between the generator 110 and the electrical load. The pressure in the pressure envelope 130 is at the intermediate pressure of the intermediate gas flow 128. This avoids the necessity for shaft seals (on the expander shaft and the compressor shaft) across a pressure drop (to the external environment) and the risk of seal failure, and allows the use of gas bearings for the turbine shaft and compressor shaft. The use of gas or magnetic bearings is advantageous as oil contamination of the gas can be minimised. The pressure envelope 130 contains natural gas, same as the incoming and outgoing streams, and residual mass transport across the bearings does not cause contamination of the gas stream.

The systems disclosed above may include the following features:

-   -   The combination of expander and heat pump using a single         rotating element.     -   Process gas used as the operating fluid for the heat pump.     -   No net electrical output from the system; a small electrical         power output from the generator (ca. 1% of expander shaft power)         can be used to run fans, with the remainder running the heat         pump.     -   The system can be hermetic with no seals to ambient, and can run         on gas bearings, precluding the potential for leaks, seal         failure or oil contamination of the gas.     -   The system can be made available as a road transportable         temporary unit, for use on site when an existing conventional         let-down system with water bath heaters fails, so removing the         requirement for a secondary stand-by unit to stand alongside         unused. This is possible due to the system being configured to         enable the heat exchanger to be appropriately sized for housing         in such a unit yet being able to sufficiently increase the gas         temperature intermediate the expander and compressor.     -   The system can have remote diagnostic and control capability         reducing or preventing site visits.

A further sub-system to reduce or virtually eliminate the frosting of the external heat exchanger is a drier that dries ambient atmospheric air before it comes into contact with the sub-zero heat exchange surfaces in order to heat the gas stream. A possible way to achieve this is to use a portion of the cold air exiting the external heat exchanger to pre-cool the air entering the heat exchanger, and dropping the temperature of the air entering the heat exchanger it to just above zero degrees to allow a large part of the condensate load to drop out. This can then be separated from the air stream. A system for drying air for a heat exchanger is described in more detail with reference to FIGS. 7 and 8.

Optionally, the system for reducing pressure in a gas flow may only be used if certain conditions are provided, for example the natural gas at the system inlet being below a certain threshold (or the ambient air temperature being below a certain threshold). For example, the system can be used only in winter, and in summer when the inlet gas temperature (and also the ambient air temperature) is relatively high the system can be switched into a summer mode with the gas being expanded directly to the outlet pipeline pressure (omitting an intermediate condition with lower pressure and temperature). At the higher temperature the gas is expanded directly to the outlet pipeline pressure and requires no heating as it remains above the minimum temperature required by the downstream system. Alternatively the system can be operated at all temperatures, and when the temperature is above a certain threshold surplus electricity can be generated.

FIG. 2c shows a variant of the system shown in FIG. 1. A first portion 131 of the gas flow 120 bypasses the expander 102 and heat exchanger 108 and the compressor 104. A second portion 124 of the gas flow is directed to the expander 102 and heat exchanger 108 and compressor 104. The first portion 131 of the gas flows via a pressure reducer 133 (such as a valve or an expander), optionally with an associated electrical generator for generating an electrical output, and is reintroduced downstream of the compressor 104. The gas flow at the outlet of the compressor 104 system provides sufficient excess heat to be mixed with the first portion 131 of the gas to produce a mixed gas stream at a suitable temperature for the downstream requirements. The second portion 124 is expanded directly to the system outlet (target) pressure, and mixed with a warmer gas stream treated by the system shown in FIG. 1 in order to ensure the resulting gas stream exiting the system is sufficiently heated.

FIGS. 3 to 6 show further versions of the system for reducing pressure in a gas flow. For the heat exchanger a blast cooler type heat exchanger is used. A recuperator can provide supplementary heat exchange. An expander turbine drives the compressor. Additionally an electrical generator is driven by the turbine and produces electricity for fans that assist the heat exchange at the blast cooler. FIGS. 3 and 4 show systems for reducing pressure in a gas flow with a relatively low let-down pressure drop, for example with a pressure drop from 44 bar at the system inlet to 38 bar at the system outlet. The intermediate pressure is for example 20 to 25 bar. The temperature drop is around 0.45 to 0.6° C. per bar pressure drop. Due to the relatively low let-down pressure drop, and assuming the expander has a suitably high pressure ratio (in order to ensure the gas temperature is sufficiently low at the heat exchanger), the compressor needs to provide relatively large compression in order to bring the gas back to the outlet pipeline pressure. Due to limited efficiency of the turbine, the turbine alone may not suffice to drive the compression in this case. Therefore the systems described with reference to FIGS. 3 and 4 provide supplemental compression drive, in addition to the drive provided to the compressor from the expander.

FIG. 3 shows a system 100 for reducing pressure in a gas flow 120. In this system 100 there is a relatively low let-down pressure drop. A first portion 122 of the gas flow bypasses the expander 102 and heat exchanger 108 and is reintroduced at the compressor 90. A second portion 124 of the gas flow is directed to the turbine expander 102 and compressor 90. The compressor 90 is a compressor with a tip turbine, and the first bypass gas flow portion 122 drives the compression of the second heated gas flow portion 124 by means of the tip turbine. In the tip turbine the first bypass gas flow portion 122 accelerates the compressor blades and provides supplemental compression drive, in addition to the drive provided to the compressor 90 from the expander 102.

A recuperator 106 allows transfer of some of the heat from the gas upstream of the expander 102 to the gas downstream of the heat exchanger 108. This can reduce the gas temperature at the heat exchanger to a sufficiently low value to enable effective heat exchange. At the heat exchanger 108 atmospheric air provides heat to warm up the intermediate gas flow 128. The warmed gas is fed into a compressor 90 and is compressed and combined with the first bypass gas flow portion 122 to form the system outlet gas flow 126 at a lower outlet pipeline pressure.

The turbine 102 drives both the compressor 90 and also an electric generator 110. The electricity generated by the electric generator 110 is used to power a fan 112 that assists the heat exchanger 108.

FIG. 4 shows a further system 200 for reducing pressure in a gas flow 120. In this system 200 also there is a relatively low let-down pressure drop. Two expanders 102 202 are combined. A first portion 222 of the gas flow is expanded in a supplementary expander 202 directly to the outlet pipeline pressure and reintroduced downstream of the compressor 104. A second portion 224 of the gas flow is directed to the turbine expander 102 and compressor 104. The supplementary expander 202 contributes supplemental drive to the drive provided from the heat pump expander 102 in order to provide sufficient drive to the compressor 104. This can enable a sufficiently low intermediate pressure (and hence sufficiently low gas temperature at the heat exchanger) for efficient heat exchange with ambient atmosphere air.

In the systems in FIGS. 3 and 4 the recuperator 106 is arranged to transfer heat from the incoming gas flow 120 upstream of the expander 102 to the intermediate gas flow 128, downstream of the heat exchanger 108 and upstream of the compressor 104. The purpose of the recuperator 106 is to reduce the gas temperature at the heat exchanger to a sufficiently low value to enable effective heat exchange with ambient air or ambient air which has been dried by cooling.

For the recuperator 106 to provide efficient transfer of heat between different gas stream portions, the temperature difference between the different gas stream portions should be sufficiently large, typically at least 5 or 10° C. In the systems with a relatively low let-down pressure drop described with reference to FIGS. 3 and 4, the temperature difference between the incoming gas flow 120 and the system outlet gas flow 126 can be relatively small and insufficient for providing heat to the inlet side, in which case the recuperator arrangement as shown in FIGS. 3 and 4 and as described above can be appropriate.

FIG. 5 shows a further system 300 for reducing pressure in a gas flow 120. In this system 300 there is a relatively high let-down pressure drop, for example with a pressure drop from 33 bar at the system inlet to 17 bar at the system outlet. The intermediate pressure is for example 9 to 12 bar. The temperature drop is around 0.45 to 0.6° C. per bar pressure drop.

A recuperator 302 allows transfer of some of the heat from the gas downstream of the compressor 104 to the gas upstream of the expander 102. This can provide pre-warming of the gas entering the expander 102. The gas flow 120 is directed to the turbine expander 102 and compressor 104.

Downstream of the compressor 104 a recirculation gas flow 322 is separated from an outlet gas flow 320. The outlet gas flow 320 passes to the recuperator 302 before progressing to the main outlet gas flow 126. The recirculation gas flow 322 is split into a bypass line 324 and a defrost line 326. The flow in the defrost line 326 is reintroduced upstream of the heat exchanger 108. Because the flow in the defrost line 326 is relatively warm it can heat the heat exchanger 108 and thus avoid frosting in the heat exchanger 108. The flow of the bypass line 324 is reintroduced downstream of the heat exchanger 108 and upstream of the compressor 104 and can be used to adjust the compressor duty.

Flow controllers 304 control the speed of the flow in the bypass line 324 and defrost line 326 based on (for example) temperature sensing or flow speed sensing. The flow controllers 304 can include an actuated valve.

In FIG. 5 typical temperatures are indicated for the different gas flows. Only one defrost line 326 is shown, but more may be included in the system 300. The defrost line 326 can defrost both internal and external heat exchanger surfaces. The duty of the heat exchanger 108 (blast cooler) in FIG. 5 is approximately 1.1 MW. The duty of the recuperator 302 in FIG. 5 is approximately 1.35 MW.

The recirculation gas flow 322 shown in FIG. 5 can be used with all system variants.

FIG. 6 shows a further system 400 for reducing pressure in a gas flow 120. In this system 400 a separator 402 is included upstream of the recuperator 302. The separator 402 separates the gas flow 120 into a liquid stream 424 and a substantially dry gas stream 422. The liquid stream 424 can include a gas and/or solid component. The substantially dry gas stream 422 passes to the recuperator 302 same as the gas flow 120 in the system 300. Actuated valves 406 control the flow in the liquid stream 424 and the gas stream 422. If necessary a heat addition 404 to the liquid stream 424 can be provided, for example by exchange with heated compressor outlet gas, or by mixing with a hot stream (with suitable restrictors).

In FIG. 6 typical temperatures are indicated for the different gas flows. The defrost line 326 can defrost both internal and external heat exchanger surfaces. Only one defrost line 326 is shown, but more may be included in the system 400. The duty of the heat exchanger 108 (blast cooler) in FIG. 6 is approximately 1.1 MW. The duty of the recuperator 302 in FIG. 6 is approximately 1.35 MW.

The separator 402 shown in FIG. 6 can be used with all system variants.

If the separator 402 is upstream of a pre-heater (where a pre-heater is used) such as the recuperator 302 shown in FIG. 6, then on entering the pre-heater the flow has very low or no liquid loading (as the liquid stream has been removed in the separator), and is a saturated gas. The heating process in the pre-heater (recuperator 302 in FIG. 6) moves the gas away from its saturation line, drying it and therefore reducing the tendency to frost downstream of the expander 102. If the separator 402 is downstream of the pre-heater, then liquid is evaporated in the pre-heater which means that the liquid stream that is removed in the separator is reduced; also, the gas exiting from the separator would be saturated, or closer to its saturation line on entering the expander 102, so more frosting would occur downstream of the expander 102. Hence locating the separator 402 upstream of the pre-heater is preferred.

In the systems in FIGS. 5 and 6 the recuperator 302 is arranged to transfer heat from the system outlet gas flow, downstream of the compressor 104, to the incoming gas stream upstream of the expander 102. In essence, this can transfer some of the heat gained from the heat exchange with ambient atmospheric air to the low pressure, low temperature portions of the gas flow in order to change the gas conditions such that condensation and especially ice formation is prevented. In some circumstances the recuperator 302 may not be necessary and can be omitted.

In a further intermediate pressure version of the system the features of a supplementary expander 202 as described with reference to FIG. 4 and a recuperator 302 arrangement as described with reference to FIGS. 5 and 6 are combined.

As can be seen from the above arrangements, the gas pressures at gas let-down stations can vary considerably depending on a variety of factors and the systems disclosed above are adaptable to the differing pressures and can be configured as discussed to efficiently reduce the pressure of a gas to a desired outlet gas pressure.

Sub-Zero Expander Outlet.

The sublimation of water vapour to ice is a non-equilibrium process so does not occur quickly enough for ice crystals to appear in the expander turbine in a volume or size to cause significant problems. Ice crystals may form in the gas downstream of the turbine, and can clog downstream equipment such as heat exchangers. The system can have defrost units for this eventuality.

Liquid formation can occur within the turbine and this is generally hydrocarbon matter that does not freeze at the temperatures considered here. Condensation may occur in the turbine wheel and not in the nozzles so erosion is avoided. It may be necessary to heat the expander shroud/body.

Blast Cooler Drier

A drier for the blast cooler (the heat exchanger) may be provided. FIG. 7 shows a system 500 for drying warm air that supplies heat to a heat exchanger 108 (blast cooler). The drier system 500 is based on an air recirculation arrangement.

The incoming air flow 520 is mixed (at Point 1 indicated in FIG. 7) with a recirculated flow portion 522 of cold air flow exiting the heat exchanger 108. This partially cools the incoming air flow 520 to around 0° C., which causes condensate to form. The condensate can then be separated in the separator 502. The resulting dried air flow 524 is conveyed to the heat exchanger 108.

By this arrangement liquid loading in the air is reduced and detrimental freezing in the heat exchanger 108 is prevented. The drier system 500 is particularly beneficial if incoming air is cooled below 0° C. in the heat exchanger 108, as frosting can cause damage and loss of performance.

A controller 504 can control the recirculation flow 522, for example by fan speed control. Temperature sensing can be used by the controller to regulate the fan speed.

In FIG. 7 typical temperatures are indicted for different parts of the flows.

FIG. 8 shows an alternative drier system 600. Instead of mixing the incoming air flow 520 at Point 1 indicated in FIG. 7 with a cold recirculated flow portion 522, heat can be transferred between the two flows 520 522 by means of a recuperator 602 upstream of the separator 502. This has the advantage of avoiding dilution of the cooling potential of the cold flow 522. Further, if a recuperator 602 transfers heat between the recirculation flow 522 and the incoming air flow 520, then the cold flow 522 entering the recuperator 602 can comprise all of the cold air exiting the heat exchanger 108, and not merely a portion of the cold air exiting the heat exchanger 108. The alternative with a recuperator 602 does not require recirculation, so the cooling potential of all of the cold air exiting the heat exchanger 108 can be harnessed before the air is discharged back to the atmosphere.

It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims. 

1. A system for reducing pressure in a gas flow for a gas let-down system comprising an expander driven by gas at a first pressure expanding to a second pressure, and a compressor for compressing the gas from the second pressure to a third pressure, whereby the third pressure is lower than the first pressure and the third pressure is higher than the second pressure. 2-40. (canceled)
 41. A system according to claim 1, wherein the expander drives the compressor, preferably directly.
 42. A system according to claim 41 wherein the expander drives the compressor by way of a common shaft.
 43. A system according to claim 1, further comprising a heat exchanger for heating the gas at the second pressure.
 44. A system according to claim 43, wherein the heat exchanger is arranged to provide heat exchange to at least one of: ambient air, to ground, or water, or an ambient heat source, and a waste heat source.
 45. A system according to claim 43, wherein the heat exchanger is arranged to provide cooling to a refrigeration load.
 46. A system according to claim 43, wherein a secondary circuit is arranged to transfer heat from a heat source to a heat exchanger.
 47. A system according to claim 1, wherein the expander further drives an electric generator.
 48. A system according to claim 1, wherein the system is arranged to conduct a portion of gas to the inlet of the expander, and to conduct a further portion of gas to the outlet of the compressor, the further portion of gas bypassing the expander and the compressor.
 49. A system according to claim 48, wherein the further portion of gas drives a further expander that optionally further drives an electric generator.
 50. A system according to claim 1, wherein the system is arranged to conduct a portion of gas to the inlet of the expander, and to conduct a further portion to the compressor, the further portion bypassing the expander.
 51. A system according to claim 50, wherein the further portion drives the compressor.
 52. A system according to claim 51, wherein the further portion drives the compressor by means of a tip turbine.
 53. A system according to claim 1, wherein the system is arranged to conduct a portion of gas to the inlet of the expander, and to conduct a further portion of gas to an inlet of a further expander.
 54. A system according to claim 53, wherein the further expander and the expander both drive the compressor.
 55. A system according to claim 53, wherein the system is arranged to conduct the further portion of gas from the outlet of the further expander to the outlet of the compressor.
 56. A system according to claim 1, further comprising a recuperator for transferring heat from one portion of the gas to another.
 57. A system according to claim 43, further comprising a recuperator for transferring heat from one portion of the gas to another and, optionally, wherein the recuperator is arranged to transfer heat from gas upstream of the expander to gas downstream of the heat exchanger.
 58. A system according to claim 56, wherein the recuperator is arranged to transfer heat from gas downstream of the compressor to gas upstream of the expander.
 59. A system according to claim 1, wherein the system is arranged to conduct a portion of gas from the outlet of the compressor to the inlet of the heat exchanger.
 60. A system according to claim 59, wherein the system is arranged to conduct a portion of gas from the outlet of the compressor to the inlet of the compressor.
 61. A system according to claim 1, further comprising a sealable vessel containing system rotative components.
 62. A system according to claim 61, wherein the system rotative components include an output drive shaft of the expander and an input drive shaft of the compressor.
 63. A system according to claim 62, further comprising a gas bearing supporting the output drive shaft of the expander or the input drive shaft of the compressor.
 64. A system according to claim 62, further comprising a magnetic bearing supporting the output drive shaft of the expander or the input drive shaft of the compressor.
 65. A system according to claim 1, further comprising a controller for activating the system when a system inlet gas temperature is below a pre-defined threshold.
 66. A system according to claim 1, wherein the expander comprises a turbine.
 67. A gas let-down station comprising a system according to claim
 1. 68. A gas distribution network comprising a system according to claim
 1. 